Electronic computing equipment currently is widely used in a variety of business activities. Businesses of all sizes are becoming increasingly dependent on computers for word processing, data processing, bookkeeping, and specialized computing applications. Computers are being used in homes to perform organizational and bookkeeping tasks. In addition, computers increasingly are being used in schools from the university level down into grade schools. The potential of electronic computers for performing data handling and data gathering tasks still is in its infancy, and the growth in the applications and extent of use of computers is explosive. Electronic computers increasingly are becoming a necessity in the performance of the day-to-day operations of business, medical facilities, schools, and homes. There is a continuous increasing demand for accuracy and reliability in the operation of these machines.
Computers or electronic data processing systems all incorporate relatively large volume memories for storing information and data. Basically, these memories fall into two different categories: namely, (1) electro-magnetic devices such as tapes, discs, bubbles, et cetera; and (2) electronic memory devices, such as RAM (random access or read and write memory) and ROM (read only memory). Electro-magnetic memory devices are non-volatile, that is, they retain data without the necessity of the presence of electric power. Once data is stored in these memories it remains stored indefinitely until such time as it may be intentionally changed. ROM memories also are non-volatile, but these memories cannot accept new information once they have been programmed. ROM memories perform pre-established special functions in the operations of computers or electronic data processing equipment.
RAM memories, which are used in all computing data processing and word processing systems, are volatile, that is, such RAM memories lose all data stored in them whenever electric power is interrupted. These RAM memories constitute the variable storage memories used in computers during the operation or processing of data, or in the creation of documents, such as letters and the like through word processors, et cetera. If electric power is interrupted or power surges cause temporary malfunction of the information in an RAM memory, all data which is being temporarily stored in such a memory can be lost or, equally as serious, errors can be introduced into the stored information without the knowledge of the operator. Consequently, it is becoming increasingly important to protect electronic computing equipment, data processors, or word processors, from total and temporary power losses and from even momentary power surges or interruptions.
A number of prior art devices have been developed for protecting the RAM memories of electronic computing equipment from power failures. In most of these systems, back-up power is provided by means of a rechargeable battery or a bank of rechargeable batteries. The most direct method is to incorporate such a battery or batteries directly into the operating system, that is to operate the system from the batteries and use the power source to maintain the battery charge. Systems of this type constitute what have become known as uninterruptible power supplied (UPS), which generally consist of a continuously-operating electronic power converter, a rechargeable battery supplying power to the converter, and a battery charger connected to the line for providing charging current to the battery. Although UPS systems offer continuous output, such systems generally are large, heavy, and expensive.
A second type of power back-up system is a standby power supply (SPS). SPS systems generally consist of a static inverter operated only when the main alternating current line power supply fails. A rechargeable battery for supplying operating power to the inverter is connected through a battery charger to the alternating current signal line, and a relay switch circuit is used to disconnect the main alternating current supply and to connect the inverter output to the electronic equipment whenever there is a power supply failure. SPS systems provide power backup with a short interruption for the switch-over. This short interruption, however, is not acceptable for some systems, even though SPS devices generally are small and economical.
Because of the small size and relatively low cost, SPS systems are preferred if the load for which such systems provide power backup can function with or tolerate the brief interruptions or abrupt phase shifts which occur when the backup system switches in or takes over from the primary system and vice-versa. A patent disclosing such a system is Servos U.S. Pat. No. 3,991,319. The Servos system is a stand-by power supply system which normally supplies power from a conventional alternating current (AC) power source to a load. A line switch is included between the AC source and the load. In addition, there is a stand-by direct current/inverter system for providing power to the load in the event there is a drop in the AC power supply or in the event such power supply is interrupted. An oscillator in the auxiliary supply is synchronized with the primary AC power. The synchronization of this oscillator is maintained during the supply of power from the AC source and it is reestablished when the AC power returns after a period of interruption. This synchronization reduces the chances of abrupt phase shifts occurring in the signal being supplied to the load whenever the power supply is switched from the main supply to the auxiliary supply, and vice-versa. The inverter in the auxiliary supply of Servos operates continuously; and a phase detector, supplied with the output of the inverter and the signal on the main alternating current supply continuously compares the two signals in phase. The phase detector produces a control signal coupled to the inverter to cause the inverter output to be in a lagging phase relationship with the primary alternating current signal. The output of the inverter and the primary alternating current signal are always connected in parallel to the output regulator which furnishes alternating current power to a load. Since the primary alternating current source is in a leading phase relationship under normal conditions of operation, the load power normally is drawn nearly entirely from the primary alternating current source. Whenever the line voltage of the primary alternating current source drops below some pre-established value, however the output of the system then automatically is supplied from the continuously operating inverter. Because of the lagging phase relationship, a phase shift is produced at the load.
Another system for providing nearly continuous alternating current power to a load is disclosed in the Hase U.S. Pat. No. 3,971,957. This patent discloses an electrical load transfer control system, the normal operating condition for which is by way of an inverter from a "float-charged" battery. The battery is constantly charged from the main alternating current supply. The battery then supplies power to the inverter which, in turn, supplies operating power to the load through a normally closed switch. Thus, this system is a type of UPS system. If there is a failure of the inverter of Hase, a fault detector circuit senses this condition and closes a switch from the main or household alternating current supply to the load, and opens the switch from the inverter. There is an intentional overlap between the switch closings and openings, but the switch-over is not necessarily entirely without any gaps or glitches.
Another patent for transferring alternating current operating power to a load between first and second sources is Studtmann U.S. Pat. No. 3,509,357. This patent is directed to a static transfer switching system having a first normally-used inverter (for producing alternating current power to a load) and a second stand-by inverter. A first switch is selectively operable to couple the normally-used source to the load; and a second switch is selectively operable to couple the stand-by source to the load. An alternating current reference signal, corresponding in wave-shape to the first source signal, is compared to the first source signal. An error signal is produced if the amplitudes of the first source and reference signals fail to correspond. This error signal then is used to effect operation of the first and second switches to connect the stand-by source to the load and disconnect the normally-used source from the load. Studtmann states that this switching takes place "substantially instantaneously". Both sources are continuously operating; and power is not supplied to the load from a standard alternating current line power supply.
It is desirable to provide a reliable and relatively low cost continuous power supply or continuous power source having the characteristics of the prior art uninterruptible power supplies (UPS) without the disadvantageous of the cost, weight, and bulk of UPS systems. It further is desirable to provide a power control or power switching circuit which does not include the abrupt phase shifts or momentary gaps typically caused at switch over from prior art stand-by supplies (SPS).